Fruits with bright and colorful peels should be peeled before consumption, as their peels are rich in "flavonoids". When these substances enter the human body, they are broken down by intestinal bacteria into compounds such as dihydroxybenzoic acid, which strongly inhibit thyroid function. In sufficient amounts, this can lead to goiter.
For example, water chestnuts grow in fertile, aquatic environments. Their skins accumulate harmful biological waste and chemical substances, so they must be peeled and cooked thoroughly before being eaten.
When persimmons ripen, tannic acid is present in their skins. In the presence of stomach acid, this substance reacts with proteins to form a precipitate known as a "bezoar" or "persimmon stone," which can cause various illnesses.